One well-known type of field effect transistor, to be mounted on a semiconductor device, in an insulated gate type field effect transistor called a MISFET (Metal Insulator Semiconductor Field Effect Transistor). A MISFET is widely used as a circuit element constituting an integrated circuit, because it facilitates high integration.
A MISFET, whether it has an n channel conductivity type or a p channel conductivity type, usually has a channel formation region, a gate insulator, a gate electrode, a source region, a drain region and the like. The gate insulator is disposed in an element formation region on the surface (one main surface) of a semiconductor substrate on which a circuit is to be formed, and it is formed, for example, of a silicon oxide film. The gate electrode is disposed, via the gate insulator, on the element formation region of the surface of the semiconductor substrate on which a circuit is to be formed, and it is formed, for example, of a polycrystalline silicon film into which an impurity has been introduced to reduce the resistance. The channel formation region is disposed in a region (right below the gate electrode) of the semiconductor substrate, opposite to the gate electrode. The source and drain regions are formed of semiconductor regions (impurity diffusion regions) disposed on both sides of the channel formation region in a channel length direction.
A MISFET having a gate insulator made of a silicon oxide film is usually called a MOSFET (Metal Oxide Semiconductor Field Effect Transistor). The term “channel formation region” means a region wherein a current channel connecting the source region with the drain region is to be formed.